Regardless, the sad but fairly well-known truth is that licenses can be revoked. Amazon content is licensed for use on the Amazon platform. Use of the Amazon platform is licensed to those with Amazon accounts. People who lose their Amazon accounts lose access to both the platform and the content as a result. I've never had a problem with Amazon (I think I've had my account since 1997) but I've also never relied on their system to protect my purchases. The same is true for B&N, Kobo, or any other DRM-participating ebook provider (and music, in Amazon's case). I like having my stuff where I can keep an eye on it.

Which is why, when I buy a book from Amazon, the first thing I do is import it into Calibre, strip the DRM, and back it up safely. I never assume that if I buy a book today, I'll be able to re-download it tomorrow.